Injury or advancing age can lead to degenerative changes in the bones, discs, joints, and ligaments of the spine, producing pain and instability. For example, spine instability may result from causes such as fracture, scoliosis, and spondylolisthesis, where one or more vertebrae moves relative to the other vertebrae. Spinal fusion is a surgical technique used to treat conditions where the spine exhibits instability by fusing together two or more vertebrae of the spinal column in order to eliminate the motion between the fused vertebrae.
The use of bone plate and bone screw fixation systems for treating injuries to bones is well established. In most instances, a bone plate may be positioned over and surrounding the bone injury area and secured to the bone. The bone plate may be secured to the bone by bone screws inserted through holes in the bone plate and into the bone itself. Intervertebral implants including interbody spacer portions and mechanically coupled plate portions can restore disc height, allow fusion to occur between the adjacent vertebral bodies, and provide stable fixation during healing. There remains a need, however, for improved devices for treating spinal instability. In particular, implants resting on the midline of the vertebrae or extending beyond the anterior face of the vertebrae can cause problems, such as irritation of the esophagus.